Remove deprecated :ssl.cipher_suites/0 call

The :ssl.cipher_suites/0 function is deprecated in favor of
:ssl.cipher_suites/2.

Since Elixir 1.11+ requires OTP 21+, we can remove the conditional
code for older OTP versions and always use the newer API.

This simplifies the code and fixes deprecation warnings:
warning: :ssl.cipher_suites/0 is undefined or private,
use cipher_suites/2,3 instead

Author: jeffkreeftmeijer

lib/appsignal/transmitter.ex

@@ -128,11 +128,8 @@ defmodule Appsignal.Transmitter do
       ]
     end
 
-    if System.otp_release() >= "20.3" do
-      defp ciphers, do: :ssl.cipher_suites(:default, :"tlsv1.2")
-    else
-      defp ciphers, do: :ssl.cipher_suites()
-    end
+    # OTP 21+ is required for Elixir 1.11+, so we always use the new API
+    defp ciphers, do: :ssl.cipher_suites(:default, :"tlsv1.2")
   end
 
   if System.otp_release() >= "21" do

test/appsignal/transmitter_test.exs

@@ -97,12 +97,7 @@ defmodule Appsignal.TransmitterTest do
       refute Keyword.has_key?(ssl_options, :versions)
       assert ssl_options[:depth] == 4
       assert ssl_options[:honor_cipher_order] == :undefined
-
-      if System.otp_release() >= "20.3" do
-        assert ssl_options[:ciphers] == :ssl.cipher_suites(:default, :"tlsv1.2")
-      else
-        assert ssl_options[:ciphers] == :ssl.cipher_suites()
-      end
+      assert ssl_options[:ciphers] == :ssl.cipher_suites(:default, :"tlsv1.2")
     end
 
     if System.otp_release() >= "21" do